This is the record of my electric gun experimentation for summer 2011. I'll gloss over the basic principles of the launcher's operation in here, but it's covered pretty well in this thread. Feel free to ask if you have any questions that aren't covered in there. Lack of time and funds, and a few minor design problems meant that I didn't get quite as much done as I'd have liked to. The following is what you'll find in this first post, subject to possible future updates:

-technical specifications of the design-description of working principles of the launcher-diagram of design-pictures of completed launcher and test setup-pictures of projectiles and targets-video clips of tests

For a stoichiometric mix, this works out to a 1:1 mixture by mass, with a density of 1.85g/cm<sup>3</sup> and an energy content of 7.8kJ/g. In the case of this launcher, the mixture used is, by mass, 5:11 (aluminum:water). Stoichiometric mixes caused incomplete combustion at lower energy inputs, and there was not sufficient time to test them fully at higher energy inputs.

Despite being very energetically favorable, this reaction is very difficult to initiate, due to the passive oxide layer which is always present on aluminum in contact with oxygen. The solution is, in this case, to boil and partially ionize it by passing an electric current through flakes of aluminum (roughly 10μm x 500μm x 500μm for this launcher) suspended in water. The current pulse is just under 40μs in duration and had a maximum magnitude between 50kA and 90kA, depending on the desired energy input. This current flow and boiling process is extremely violent and induces essentially instant mixing and combustion, converting a room temperature fluid-solid slurry into a 6000K plasma, which is then used as the propellant gas in the same manner as in a conventional combustion gun. Pressure generated is confirmed to be in excess of 700MPa (100kpsi or 6800 bar), and the performance with lightweight rounds appears to indicate that the hydrogen:metal oxide ratio at the projectile base increases dramatically during the shot, making the effective sound speed much higher than would be expected.

Regrettably, no direct measurement of performance was possible this summer (we're working on it though, and it should be ready for next time around). This propulsion method, while capable of high speeds, is by its nature rather inefficient at all but *very* low speeds (all that hot aluminum oxide is essentially wasted energy). Lack of available energy limited the performance achieved, and it may not be possible to properly test the very high speed end of things with my current capacitor bank (although I could, in theory, increase the current pulse to 24kJ, the capacitors' operational lifespans would likely be measured in tens of shots).

Assembled barrel (the threaded section is a 3/4"-16 B7 threaded rod). Two impact marks are visible here: the large yellow one is from a brass ball at 16kJ chamber energy, the small circle is spalling from an airsoft round hitting the other side of the plate (at 11.5kJ chamber energy). Higher velocity capabilities can be achieved by increasing the ratio of the electric pulse energy to the chamber chemical energy.

The worst leaks occurred at the chamber-breech section. These should be remedied by modifying the sealing face so that the insulator fits inside the breech, rather than the other way around (this causes the insulator to expand under pressure and seal against the breech, a scheme that worked very well in the old ETA). Another seal that leaked somewhat was the breech-barrel interface. This is also easily remedied by adding a small brass gasket (more like a very short tube) which seals in the same way as mentioned for the insulator (this is how firearm cartridges work, and with close tolerances they are reusable for many shots).

The bolted connections may be replaced with threads after I get the tooling needed for that modification, and the thread engagement on the insulator may be increased for very high pressure shots (small chamber volume, high electric discharge energy). Also in the works is a smoothbore barrel to replace the current "roughbore" version

This post is not complete - there's a good deal of relevant information that just didn't make the cut to get into this summary. Feel free to ask questions, and I'll do my best to answer them. Ideas for improvement are, as always, greatly appreciated.

* Unless you're a master of the "advanced science", and the sort of genius who can figure out how to make a combustion gun that runs on GASOLINE. In that case, I bow before your vast intellect, and beg you to devote your mighty powers of invention to justice and peace

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Last edited by DYI on Sat Sep 17, 2011 12:06 pm, edited 3 times in total.

Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.

Why didn't you go for extending the plastic sleeve instead of the jelly?

Drilling out the ABS rods to make the insulators is a royal PITA to start with, and because of surface arcing, they'd have to be MUCH longer to work without the jelly. My tap is only 6" long (the current insulators are 5.5"), so the extension would have to be drilled out to 3/8" as well. Coating the insulator with oil and then slipping a separate plastic tube over the end would likely work, but this thing takes so long to reload that it'd hardly make a difference.

Also: Yes Hotwired, it is Apocalyptica on their cellos.

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Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.

It looks cool and all, but wouldn't it shoot better if you made the chamber more ergonomic and injected gasoline? I did some advanced science and found that gasoline burns better than water.

All joking aside, this is easily one of, if not the most impressive launcher ever posted on Spudfiles. Very nice work.

I was going to suggest modifying the geometry of the internal mating surfaces to create an expansion seal at the interface between the insulator and the breech, as well as the barrel to breech connection, but you've already thought of that. Otherwise, the only significant improvement might be a reduction in reload time, but we've already discussed that issue fairly extensively in the chat.

Oh, and I almost forgot, excellent job on the video.

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People should not be afraid of their governments. Governments should be afraid of their people.

This is clearly a launcher of a different color. Very impressive as others have stated. What I find equally as impressive is that you had the follow through and motivation to actually persue this until it worked.